1. Field of the Invention
The present invention relates to a method for designing an optical pickup incorporated in an optical disc apparatus.
2. Description of Related Art
An optical disc apparatus that plays back from and records to an optical disc such as a CD or DVD incorporates an optical pickup. Conventionally, different optical pickups have been developed to cope with recording to and playback from different types of discs.
For example, a DVD recorder employs an optical pickup that can handle recording to a DVD and playback from a CD. FIGS. 1 and 2 show the optical system of an optical pickup for a DVD recorder, FIG. 1 showing it as viewed from the side and FIG. 2 showing it as viewed from the top. It should be noted that the coordinate axes (X, Y, and Z) shown in these diagrams are common to other diagrams wherever applicable.
A laser diode 1 emits laser light. The structure of the laser diode 1 is shown in FIG. 3. A submount 5 is fixed on a projection 4 formed to project from a disc-shaped stem 3. In an upper end part of the submount 5, a monolithic laser diode (hereinafter “monolithic LD”) 6 is arranged. Moreover, on the submount 5, behind the monolithic LD 6, a PIN diode 7 is formed. The monolithic LD 6 emits laser light of different wavelengths, namely at a DVD wavelength (in a 650 nm band) and at a CD wavelength (in a 780 nm). To achieve control, called APC (automatic power control), for keeping the output of the laser light emitted from the monolithic LD 6 constant, the PIN diode 7 receives the laser light emitted rearward from the monolithic LD 6. The stem 3 is penetrated by a plurality of terminals 2, which are connected by leads 8 to the monolithic LD 6 and to the PIN diode 7, which thus receive their drive current via the terminals 2.
FIG. 4 shows the monolithic LD 6 as viewed from the direction in which laser light is emitted (from the direction indicated by arrow L in FIG. 3). The monolithic LD 6 has, integrated into a single chip, a laser structure capable of emitting at two wavelengths, namely at a DVD wavelength (in a 650 nm band) and at a CD wavelength (in a 780 nm). The monolithic LD 6 has a common negative electrode 9, a GaAs substrate 10, an active layer 11, a DVD-side p electrode 14, and a CD-side p electrode 15. On the GaAs substrate 10, the individual layers including the active layer 11 are laid (downward in the diagram) on one another, and, to the lowermost layer thereof, the DVD-side p electrode 14 and the CD-side p electrode 15 are connected. In the active layer 11, a DVD-side laser emitting portion 12 and a CD-side laser emitting portion 13 are formed. To the DVD-side p electrode 14, a DVD-side positive electrode 16 formed on the submount 5 is connected, and, to the CD-side p electrode 15, a CD-side positive electrode 17 formed on the submount 5 is connected. Moreover, to the GaAs substrate 10, the common negative electrode 9 is connected. When electric current is passed through the electrodes, the DVD-side laser emitting portion 12 emits laser light (in a 650 nm wavelength band) for DVD recording, and the CD-side laser emitting portion 13 emits laser light (in a 780 nm wavelength band) for CD playback. The laser light for DVD recording has a higher output than the laser light for CD playback. The positions of the DVD-side laser emitting portion 12 and the CD-side laser emitting portion 13 are determined by a semiconductor wafer process, and hence with high accuracy.
Emitted from the laser diode 1 structured as described above, the laser light for DVD recording or CD playback is then split, by a grating 18, into one main beam and two sub beams. The laser light then passes through a PBS (polarizing beam splitter) 19 and then through a quarter-wave plate 20, is then reflected on a upward-directing mirror 21, and then enters a collimator lens 22. By the collimator lens 22, the laser light is formed into parallel light, and then the laser light enters an aperture 23. The aperture 23 is wavelength-selective so as to let the laser light for DVD recording (in a 650 nm wavelength band) pass therethrough intact but restrict the aperture for the laser light for CD playback (in a 780 nm wavelength band). The laser light that has passed through the aperture 23 is focused on a recording surface of a disc 25 by an objective lens 24.
The laser reflected from the disc 25 passes through the objective lens 24, then through the aperture 23, and then through the collimator lens 22, and is then reflected on the upward-directing mirror 21. The laser light then passes through the quarter-wave plate 20, and then enters the PBS 19. The laser light that enters the PBS 19 here has passed through the quarter-wave plate 20 twice, and is therefore now reflected by the PBS 19 to enter a cylindrical lens 26.
As shown in FIG. 5A, the cylindrical lens 26 has a concave cylindrical surface, and is arranged so that this concave cylindrical surface faces the PBS 19. Moreover, as shown in FIG. 5B, which shows the cylindrical lens 26 as viewed from the direction indicated by arrow A in FIG. 2, the cylindrical lens 26 is arranged so that its central generatrix S lies on an XY plane and is inclined 45° from the X direction.
Having passed through the cylindrical lens 26 described above, the laser light is then received by a photodetector 27. FIG. 6 shows the photodetector 27 as viewed from the direction indicated by arrow B in FIG. 2. The photodetector 27 has, formed on a single silicon substrate 28, a plurality of photodetective portions 29 to 34 that cope with laser light of two wavelengths. More specifically, one photodetective portion 30 having its photodetective surface divided into four parts receives the main beam of the laser light for DVD recording; two photodetective portions 29 and 31 having their photodetective surfaces each divided into two parts receive the sub beams of the laser light for DVD recording; one photodetective portion 33 having its photodetective surface divided into four parts receives the main beam of the laser light for CD playback; and two photodetective portions 32 and 34 having their photodetective surfaces undivided receive the sub beams of the laser light for CD playback. These photodetective portions individually convert the laser light they have received into electrical signals, based on which the RF signal recorded on the disc is reproduced and a focus error signal and a tracking error signal are produced.
Inconveniently, however, the optical pickup configured as described above has the following disadvantages.
FIG. 7 is a diagram showing how the monolithic LD 6 described above emits laser light. The intensity distribution of the laser light emitted from the DVD-side laser emitting portion 12 or the CD-side laser emitting portion 13 reflects how the laser emitting portion itself is shaped in the active layer 11, and thus has an elliptic pattern elongate in the direction perpendicular to the active layer 11. The intensity distribution of the laser light exhibits a Gaussian distribution both in the directions parallel and perpendicular to the active layer 11, and the angle encompassing the part of such a distribution pattern where the light intensity is equal to or higher than a predetermined proportion (for example, one half) of its peak value is called the radiation angle. In the following description, the radiation angle (θ// in FIG. 7) in the direction parallel to the active layer 11 will be referred to as the parallel radiation angle, and the radiation angle (θ⊥ in FIG. 7) in the direction perpendicular to the active layer 11 will be referred to as the perpendicular radiation angle. FIG. 8 shows the light intensity distribution in the direction perpendicular to the active layer 11, as observed with the laser light for CD playback and the laser light for DVD recording, the latter having a higher output than the former. With the laser light for DVD recording, which has the higher output, the perpendicular radiation angle is small, resulting in a sharper distribution pattern; by contrast, with the laser light for CD playback, which has the lower output, the perpendicular radiation angle is large, resulting in a gentler distribution pattern.
Here, as shown in FIG. 9, the direction (hereinafter referred to simply as “emission direction) in which the intensity of the laser light emitted from each emitting portion of the monolithic LD 6 has the peak value may deviate, in the direction perpendicular to the active layer 11 of the monolithic LD 6, from the reference axis along the optical path leading from each emitting portion of the monolithic LD 6 to the center of the photodetective portion 30 (for the DVD main beam) or the photodetective portion 33 (for the CD main beam) of the photodetector 27. In the following description, the angle of this deviation will be referred to as the emission angle (Δθ⊥ in FIG. 9). This deviation is attributable to an error in the emission direction at each emitting portion resulting from a fabrication error in the monolithic LD 6, and to an error in the fitting of the monolithic LD 6 itself resulting from an error in the submount 5 or the stem 3.
The intensity distribution of the laser light immediately after exiting from the objective lens 24, as observed when the emission direction deviates downward in the X-axis direction from the reference axis as shown in FIG. 9, is shown in FIG. 10A. Moreover, with respect to both of a case where the emission direction is not deviated from the reference axis and a case where it is deviated as shown in FIG. 9, the cross-section, as viewed in the Z-axis direction, of the intensity distribution of the laser light immediately after exiting the objective lens 24 is shown in FIG. 10B. In FIG. 10, the symbol “Dob” represents the beam diameter of the laser light immediately after exiting the objective lens 24. As these diagrams show, when the emission direction of the laser light deviates in the X-axis direction from the reference axis, the intensity distribution of the laser light immediately after exiting the objective lens 24, as having been restricted by the aperture 23 and the member that keeps the objective lens 24 in position, becomes lopsided in the Y-axis direction, and thus the gravity center of the light intensity deviates in the Y-axis direction.
As the light intensity of the laser light immediately after exiting the objective lens 24 deviates in this way, so the light intensity distribution at the photodetective portions of the photodetector 27 deviates. With respect to a case where the emission direction deviates downward in the X-axis direction from the reference axis as shown in FIG. 9, the light intensity distribution and the gravity center (indicated with the symbol “×”) of the light intensity as observed at the photodetective portion 30 (for the DVD main beam) and the photodetective portion 33 (for the CD main beam) are shown in FIG. 11A. As this diagram shows, when the emission direction of the laser light deviates in the X-axis direction from the reference axis, the light intensity distribution at the photodetective portions becomes lopsided in the Y-axis direction, and thus the gravity center of the light intensity deviates in the Y-axis direction from the center of the photodetective portions.
Here, as shown in FIG. 11A, let the four photodetective parts of the photodetective portion 30 be represented by “a”, “b”, “c”, and “d”, and let the four photodetective parts of the photodetective portion 33 be represented by “A”, “B”, “C”, and “D”. Then, the photodetector 27 needs to be adjusted so that the light reception balances expressed by formulae (1) and (2) below equal, ideally, zero.PDY1=((Ia+Ib)−(Ic+Id))/(Ia+Ib+Ic+Id)×100  (1)PDY2=((IA+IB)−(IC+ID))/(IA+IB+IC+ID)×100  (2)where PDY1 (%) represents the light reception balance at the DVD photodetective portion; PDY2 (%) represents the light reception balance at the CD photodetective portion; and I represents the light intensity at the photodetective part i.
The state that appears when an adjustment is made in the state shown in FIG. 11A by moving the photodetector 27 in the Y-axis direction so that the gravity of center of the light intensity in the DVD photodetective portion 30 is located at the center of the photodetective portion 30 is shown in FIG. 11B. This makes the light reception balance (formula (1) above) in the DVD photodetective portion 30 equal to zero, but leaves the gravity center of the light intensity in the CD photodetective portion 33 deviated from the center of the photodetective portion 33, and accordingly leaves the light reception balance there (formula (2) above) non-zero. This is called the remnant light reception balance deviation. Due to errors in the emission direction of the emitting portions of the monolithic LD and errors in the fitting of the monolithic LD itself, the laser emission angle varies from one individual optical pickup to another, and, depending on the magnitude of the emission angle, the deviation of the gravity center of the light intensity in the state shown in FIG. 11A may be so great that, after the adjustment of the photodetector, the just-mentioned remnant light reception balance deviation exceeds the tolerable range. When the remnant light reception balance deviation exceeds the tolerable range, the reading of the disc and the servo operation are adversely affected, and this necessitates a further adjustment. Since the remnant light reception balance deviation cannot be eliminated through an adjustment involving the rotation of the photodetector, however, the optical pickup is then evaluated as defective, and this leads to a lower yield. Instead, an adjustment may be made in part of the optical system other than the photodetector, but this leads to increased cost.
Incidentally, JP-A-2003-22543 discloses a method for adjusting light reception balance whereby a deviation of light reception balance attributable to a deviation of the direction of laser emission from an LD from the reference axis running in the direction parallel to the active layer thereof is adjusted by inclining the LD.